Piezoelectric resonators are well known in the art. Piezoelectric resonators are electronic elements used to make a frequency stable and selectable. Piezoelectric resonators are widely used in various kinds of electronic equipment including communication systems, intelligence sensors, precision guided munitions, cordless telephones, broadcast and television, satellite telecommunication, electronic clocks, digital instruments and so on. Piezoelectric resonators can also be used as sensors of temperature, pressure and weight. The properties of the crystal resonator depend on the angles of cut. Metal electrodes are disposed upon the crystal wafer, which is mounted in a structure designed to hold the crystal wafer. This crystal and holder assembly is called a piezoelectric resonator. Piezoelectric crystal devices are used primarily for precise frequency control and timing. Quartz is the most widely used piezoelectric material. Quartz resonators are manufactured by cutting wafers from the mother crystal along precisely controlled directions with respect to the crystallographic axes. A quartz crystal acts as a stable mechanical resonator, which, by its piezoelectric behavior and high Q, determines the frequency generated in an oscillator circuit. Bulk-wave resonators are available in the frequency range from about 1 kHz to 200 MHz. Surface-acoustic wave (SAW), shallow-bulk-acoustic-wave (SBAW), and surface transverse wave (STW) devices can be made to operate at well above 1 GHz.
The frequency of a piezoelectric resonator is adversely affected by environmental stresses that deform the resonator, including vibration and shock, gravitational stress, temperature, aging, thermal hysteresis and so on. Even the acceleration due to gravity produces measurable effects and the frequency of a piezoelectric resonator can shift significantly when turned upside down due to gravity. For example, when an oscillator using an AT-cut crystal is turned upside down, the frequency typically shifts about 4×10−9 and acceleration sensitivity of an AT-cut crystal is typically 2×10−9 g−1. The sensitivity is the same when the crystal is subjected to vibration, i.e., the time-varying acceleration due to the vibration modulates the frequency at the vibration frequency with amplitude of 2×10−9 g−1. In the frequency domain, the vibration sensitivity manifests itself as vibration-induced sidebands that appear at plus and minus the vibration frequency away from the carrier frequency. The acceleration sensitivity of SC-cut crystals can be made to be substantially less than that of comparably fabricated AT- or BT-Cut crystals.
The stresses caused by acceleration, vibration and shock are well-known to those skilled in the art. Periodic acceleration in the form of vibration can cause frequency modulation in piezoelectric resonators, and shock can cause a step frequency change in a piezoelectric resonator due to the resonator's acceleration sensitivity. Shock can also cause a permanent frequency change in a piezoelectric resonator if either the supporting structure or the electrodes are stressed beyond their elastic limits. If during shock the elastic limits in the crystal's support structure or in its electrodes are exceeded, the shock can produce a permanent frequency change. Crystal units made with chemically polished plates can withstand shocks in excess of 20,000 g. Such crystals have been successfully fired from howitzers; however this ability to withstand shock is not typical. Therefore the stresses caused by acceleration, vibration and shock and the consequent negative and deleterious effects on piezoelectric frequency stability have caused prior art piezoelectric resonators to suffer from numerous disadvantages, limitations and shortcomings.
Similar difficulties are encountered in undamped mechanical mounting systems where the dynamic amplification at the mechanical resonance frequency of the support structure causes unwanted and undesirable frequency shifts in the piezoelectric resonator. Prior art approaches to the problem of undamped mechanical mounting systems have typically involved large packages allowing for large travel of the support structure and a degradation of specifications near the mechanical resonance frequency. However, these techniques have proven unsatisfactory due to a number of disadvantages, limitations and shortcomings such as excessive size and weight, and the need for improved performance at low mechanical vibration frequencies. Thus, there has been a long-felt need to overcome the problems caused by the dynamic amplification of an unrestrained, or undamped, piezoelectric resonator at the mechanical resonance frequency. Until now, there are no currently available piezoelectric packaging techniques that satisfactorily damp mechanical and structural resonances of a mounted piezoelectric device.